Solidification Structures in Overlaying of Cobalt-Based Superalloy on Ferritic Spheroidal Graphite Cast Irons by Plasma Transferred Arc Process

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When spheroidal graphite (SG) cast iron is overlaid with Stellite 1 cobalt-based superalloy by a plasma transferred arc melting process, a carbide-containing interfacial layer can exist between the substrate and the overlayer. Meanwhile, ledeburite can occur in the partially melted zone of the substrate. These microstructures may cause damage during application and thus are undesirable. In the present investigation, Stellite 1 alloy was overlaid on seven ferritic SG cast irons of various silicon content (2.0 to 3.9 mass%) and carbon content (1.5 to 3.8 mass%) under the same overlaying condition (current: 60 A; voltage: 26 V). The purpose was to characterize the solidification structure with special emphasis on the interface region and the partially melted zone. The results indicate that the overlayer is hypereutectic. The carbide-containing interfacial layer can be eliminated by reducing the carbon content of the substrate. Both reducing the carbon content and increasing silicon content can reduce the amount of ledeburite formed in the partially melted zone. The formation of the ledeburite can be avoided if the silicon content is sufficiently large.